1. Field of the Invention
The invention relates to a membrane separation process and a membrane plant for energy-efficient oxygen generation using mixed conducting ceramic membranes.
2. Discussion of Background Information
At the present time, conventional production of oxygen is preferably carried out through pressure swing adsorption (PSA) or cryogenic air separation (Linde® process). Large scale plants which are highly energy-optimized attain specific energy consumptions of a minimum of 0.34 kWhel./Nm3 O2 (cryogenically: Fu, C., Gundersen, T., “Using exergy analysis to reduce power consumption in air separation units for oxy-combustion processes”, Energy 44 (2012) 1, 60-68) or 0.36 kWhel./Nm3 O2 (PSA: Dietrich, W., Scholz, G., Voit, J., “Linde-Verfahren zur Gewinnung von Sauerstoff und Ozon für Zellstoff-und Papierfabrik [Linde process for obtaining oxygen and ozone for a pulp and paper mill]”, Berichte aus Technik und Wissenschaft 80 (2000), 3-8). However, this specific energy consumption of conventional plants increases sharply with the aimed-for purity of the oxygen product gas and with decreasing plant size. Accordingly, smaller PSA plants with an output of up to approximately 1000 Nm3 O2/h need at least 1.0 kWhel./Nm3 O2, but only deliver 95 percent by volume of oxygen. Owing to the high specific energy consumption, a decentralized oxygen generation is not economically feasible for many applications in combustion and gasification technology. Supply via flasks or liquid tanks, particularly with continuous oxygen requirement, is even less economical.
An alternative method for the production of oxygen is based on a membrane separation process at high temperatures. Mixed conducting ceramic membranes (MIEC—Mixed Ionic Electronic Conductors) are used for this purpose and enable a highly selective separation of oxygen. The oxygen transport relies on the transporting of oxide ions through the gastight ceramic material and the transporting of electronic charge carriers (electrons or electron holes) taking place simultaneously. Since the 1980s, a large number of ceramic materials have been investigated with respect to oxygen transport and further material characteristics (Sunarso, J., Baumann, S., Serra, J. M., Meulenberg, W. A., Liu, S., Lin, Y. S., Diniz da Costa, J. C., “Mixed ionic-electronic conducting (MIEC) ceramic-based membranes for oxygen separation”, Journal of Membrane Science 320 (2008), 13-41).
Oxygen permeation through an MIEC membrane can be described by Wagner's equation and is determined primarily through the ambipolar conductivity of the material at operating temperature, the membrane thickness and the driving force. The latter is given by the logarithmic ratio of oxygen partial pressure in the feed gas pO(h) to oxygen partial pressure in the sweep gas (pO(l) or in the permeate. Consequently, in a given material with constant membrane thickness and fixed temperature, the oxygen flux through a MIEC membrane is proportional to In{pO(h)/pO(l)}. Accordingly, doubling pO(h) on the feed gas side results in the same increase in oxygen flux as halving pO(l) on the permeate side or sweep gas side. Consequently, in order to generate pure oxygen in plants utilizing membrane technology, the air can be compressed or the oxygen can be sucked out by vacuum. Of course, combined processes are also possible (Armstrong, P. A., Bennett, D. L., Foster. E. P., Stein. V. E., “ITM Oxygen: The New Oxygen Supply for the New IGCC Market”, Gasification Technologies 2005, San Francisco, 9-12 Oct. 2005). Compression of air is preferred for commercial plants inter alia because compressors are, inter alia, generally cheaper and more available than vacuum generators.
The technological feasibility of oxygen generation with MIEC membranes has already been demonstrated on a small scale through the construction and operation of an electrically heated, portable oxygen generator with electrically operated vacuum pump (Kriegel, R., “Einsatz keramischer BSCF-Membranen in einem transportablen Sauerstoff-Erzeuger [Use of ceramic BSCF membranes in a portable oxygen generator]”, J. Kriegesmann (ed.), DKG Handbuch Technische Keramische Werkstoffe, Loseblattwerk, HvB-Verlag Ellerau, 119. Erg.-Lieferung, November 20120, Chapter 8.10.1.1, pages 1-46). However, at 1.6 kWh/Nm3 O2, the specific energy consumption of the device described therein was appreciably higher than in the conventional processes; moreover, the thermal energy requirement was not taken into account.
The own energy requirement for MIEC membrane separation results on the one hand from the thermal energy required for maintaining the high temperature of 800-900° C. at the membrane. On the other hand, compression energy for gas compression is needed to generate the driving force for oxygen transport. If the air is compressed on the feed side, it is necessary to expand the compressed, O2-depleted air via a gas turbine in order to recover the expended compression energy. As an alternative to this overpressure process, the oxygen can be obtained through vacuum suction. The vacuum process requires less compression energy, but this compression energy cannot be recovered. Corresponding processes have already been described a number of times in the field of power plant engineering (WO 2008/014481 A1, EP 2 067 937 A2, WO 2009/065374 A3, EP 2 026 004 A1). Only WO 2009/065374 A3 claims a vacuum process.
In the power plant domain, the own energy requirement of MIEC membrane plants is influenced considerably by integration into the power plant. Accordingly, depending upon the degree of integration of the MIEC membrane plant, the calculated own energy requirement for the overpressure process fluctuates between 0.031 and 0.134 kWhel./Nm3 O2 (Stadler, H., Beggel, F., Habermehl, M., Persigehl, B., Kneer, R., Modigell, M., Jeschke, P., “Oxyfuel coal combustion by efficient integration of oxygen transport membranes”, International Journal of Greenhouse Gas Control 5 (2011), 7-15). An energy requirement of a minimum of 0.14 kWhel./Nm3 O2 has been specified for the vacuum process (Nazarko, J., Weber, M., Riensche, E., Stolten, D., “Oxygen Supply for Oxyfuel Power Plants by Oxy-Vac-Jül Process”, 2nd International Conference on Energy Process Engineering, Efficient Carbon Capture for Coal Power Plants, 20-22 Jun. 2011, Frankfurt/Main). However, other authors have found no noticeable difference for the membrane process for cryogenic air separation (Pfaff, I., Kather, A., “Comparative Thermodynamic Analysis and Integration Issues of CCS Steam Power Plants Based on Oxy-Combustion with Cryogenic or Membrane Based Air Separation”, Energy Procedia 1 (2009) 1, 495-502). These sharply varying or contradictory results which were obtained under widely different boundary conditions are obviously not suitable for energy assessment for an autonomous membrane plant without coupling to a power plant.
In the studies mentioned above, modeling calculations are carried out with complex software tools to identify and debate dependencies of the own energy requirement of the membrane process on air throughput, degree of separation of oxygen from the supplied air (feed gas), and procedural integration into the power plant. However, no simple, comprehensible relationship between influencing parameters and the specific energy consumption of a MIEC membrane plant has been specified or deduced because modeling is always carried out in the context of linking to the power plant. Accordingly, it has not been possible heretofore to predict the optimal operating point for a planned MIEC membrane plant at a reasonable expenditure or, consequently, to configure all components to this optimal operating point.
According to the prior art, the area-normalized oxygen permeation of the membrane material is considered crucial for economical operation of a MIEC membrane plant. Consequently, a minimum oxygen permeation of 10 Nml(cm2·min) has been postulated for economical operation (Vente, Jaap F., Haije, Wim. G., Ijpelaan, Ruud, Rusting, Frans T., “On the full-scale module design of an air separation unit using mixed ionic electronic conducting membranes”, Journal of Membrane Science 278 (2006), 66-71). Consequently, current work for developing MIEC membranes is almost entirely oriented to the highest possible oxygen permeation (Baumann, S., Serra, J. M., Lobera, M. P., Escolástico, S., Schulze-Küppers, F., Meulenberg, V. A., “Ultrahigh oxygen permeation flux through supported Ba0.5Sr0.5Co0.8Fe0.2O3-δ membranes”, Journal of Membrane Science 377 (2011) 198-205). High feed-throughputs and pure oxygen as feed are used for this purpose; the influence of O2 depletion in the feed on O2 permeation and on the energy demand is not taken into account. A comprehensive assessment of energy consumption of autonomous, self-contained MIEC membrane plants has not been achieved to date.